Those skilled in the art know that fatigue is considered the major cause of bearing failure. The rating life of a sufficiently large group of apparently identical bearings is often defined as a number of revolutions that 90 percent of those bearings will complete or exceed before the first evidence of fatigue develops. This method assumes an exact alignment of the two members of the bearing which rotate relative to each other. For all practical purposes a perfectly aligned bearing cannot be achieved. Since one part of the bearing can be considered to be fixed relative to the other, angular mis-alignment between the two parts leads to a periodic overloading condition with a subsequent reduction in bearing life. In many rotating gear or reduction gear transmissions a shaft is supported at each end by a bearing. If the shaft is not perfectly aligned each bearing is not evenly loaded. When that shaft carries a driven component, such as a spur gear, the load imposed upon that component can result in the shaft's axis being periodically deflected or mis-aligned relative to the alignment of the two supporting bearings. The longer and more slender the shaft the more likely that this will occur. Thus, bearing life is not only affected by inaccurate machining of the bearing parts themself but also by the alignment of the bearing housing, or support, the misalignment of the rotating components, and the forces imposed on the bearing by these devices rotationably connected to the components supported by the bearing. It has been estimated that bearing life can be reduced by as much as 20 percent if that bearing is misaligned to the largest or to the greatest extent usually permitted by a bearing manufacture's published specifications.
In an elementary planetary transmission, star gears (compound type) are mounted in or on a fixed carrier assembly. A sun gear at the interior of the carrier drives the star gears and another sun gear is driven by the star gears (i.e., a so-called "sun - sun transmission). In addition to the sun - sun transmission there is the star compound configuration and the differential transmission. In the star compound, the axis of the star gears are fixed and the star gears drive a rotating ring gear coupled to the output shaft of the transmission. In the differential transmission the star gears are mounted within a rotating carrier (i.e., the star gears become planetary gears) with one end of each planetary gear rotating within a fixed ring gear and with the other end driving a rotating ring gear which is coupled to the output shaft of the transmission. Clearly, if there are alignment errors with respect to the axes of the gears and the axes of the supporting bearings, the load imposed is not uniform. This not only leads to an eventual fatigue failure but also to excessive abrasion, vibration and noise. More importantly this mis-alignment can be manifested by a power loss across the transmission.
There have been several attempts to resolve this loading problem. For example, Campbell in U.S. Pat. No. 4,092,878 employs a floating carrier. Shipitalo in U.S. Pat. No. 3,583,252 employs a series of additional intermediate gears and sun gears which are roller gears. Takahashi in U.S. Pat. No. 4,237,750 uses a "floating intermediate ring" to enhance the spring action of an oil film thereby improving the load balance on a set of planetary gears. Others have approached the problem by using unique gear shapes: U.S. Pat. Nos., 4,280,376; 4,115,022; 3,331,217; 3,232,075; 2,922,294; 2,687,025; and 2,114,807. Flexible couplings have also been suggested: U.S. Pat. Nos. 2,380,113; 2,841,966; and 3,243,973. Clearly those skilled in the art have not found a universally satisfactory solution to the mis-alignment problem. More importantly a simple and otherwise innovative and practical approach to this design problem is needed.